Module 4

Basic capacity (flow control systems)

IntroductionDischarge ThrottlingSuction ThrottlingAdjustable Inlet Speed Control

Introduction

There are basically two main methods for compressor control i.e. capacity control systems and antisurge control systems. In this module we will explain the capacity control systems while the antisurge control systems will be explained in details in chapter (5). The choice of capacity control system depends on process system characteristics, compressor characteristics and the overall efficiency considerations. Capacity control can be obtained by one of the following methods.

Discharge Throttling

Figure 4-1 shows the use of discharge throttling to control the suction flow or the discharge pressure of the compressor. To decrease suction flow or increase discharge pressure, the discharge valve would be stroked further closed, this corresponds to the demand load curve raising, and the intersection of this curve and the compressor characteristic curve moving up from L1 to L2 in Figure 1-3 (module 1) for a centrifugal compressor and in Figure 1-5 (module 1) for an axial compressor. The operating point moves along a single characteristic curve. Thus the flow cannot be decreased or the pressure increased too much without the operating point approaching too close to the surge curve.

Limits would have to be placed on either the controller output or the valve position to prevent the valve from closing too much and causing surge. Discharge throttling is also the least efficient of the throughput control methods because the energy loss is large due to the large pressure drop across the valve. Discharge throttling is rarely used due to this poor turndown and efficiency.

Figure 4-1 Discharge Throttling Schematic

To decrease suction flow or increase discharge pressure, the discharge valve would be stroked further closed. The operating point moves along a single characteristic curve. Thus the flow cannot be decreased or the pressure increased too much without the operating point approaching too close to the surge curve. Limits would have to be placed on either the controller output or the valve position to prevent the valve from closing too much and causing surge. Discharge throttling is not efficient as energy loss is large (valve DP). Discharge throttling is rarely used due to its poor turndown and efficiency.

Suction Throttling

Figure 4-2 shows the use of suction throttling to control the suction flow or the discharge pressure of the compressor to decrease suction flow or decrease discharge pressure, the discharge valve would be stroked further closed, the pressure drop across the suction valve is much smaller than that across the discharge valve, and the closing of the suction valve is reduces the inlet gas density, which reduces the horsepower requirement. Consequently suction throttling is more efficient than discharge throttling.

Figure 4-2 suction throttling schematic

Adjustable Inlet Guide Vanes

Figure 4-3 shows the use of guide vanes to control the suction flow or the discharge pressure of the compressor. To decrease suction flow or decrease the discharge pressure, the guide vanes should be stroked further closed. A change in the guide vane position changes the suction pressure and the amount of pre-rotation of the gas. The vane provides a maximum counter-rotation of the gas at their minimum position. The use of pre-rotation increases the efficiency at low loads. Guide vane positioning is more

efficient than suction throttling and has the greatest turndown capability of all the throughput control methods.

Figure 4-3 Guide vane positioning schematic

Speed Control Figure 4-4: shows the use of speed control to control the suction flow or the discharge pressure of the compressor. The speed control loop is the inner loop of a cascade control system. The set point of the speed controller is the output of either the flow or the pressure controller. The type of speed control depends whether the driver is a turbine or a motor. The output of the speed controller can either throttle the steam or gas flow to a turbine, as depicted in Figure 4-4, or vary the electrical frequency of the power to the motor.

To reduce the suction flow or the discharge pressure, the speed would be reduced by decreasing the turbine inlet flow or the motor power frequency.

Therefore, the reduction in throughput by a reduction in speed is the most efficient method. Since the surge curve bends up for speed control and over for vane position control, the operating point is closer to the surge point at low flow, and the turndown capability of speed control is less than that for guide vane positioning. The speed control range cannot be extended so low that it includes critical speeds. The operating point must pass through critical speeds as fast as possible on start up to avoid damage due to high vibration.

Figure 4-4 Speed Control Schematic

The speed control loop will control the suction flow or the discharge pressure of the compressor. The speed control loop is the inner loop of a cascade control system.The set point of the speed controller is the output of either the flow or the pressure controller. To reduce the suction flow or the discharge pressure, the driver speed would be reduced. The reduction in throughput by reducing the speed is the most efficient method. The speed control range cannot be extended so low that it includes critical speeds.

Figure 5-5 shows the operating range of compressor speed in RPM and of compressor inlet flow in acfm for different types of drivers. Whether a motor or a turbine driver is used depends also upon the availability and the cost of steam or gas versus power within the plant.

Figure 5-5 Compressor Driver Operating Ranges

Selection of the suitable driver could be based on the following parameters

a) The operating range of compressor speed in rpm and of compressor inlet flow in acfm for different type of drivers as shown above.

b) Availability and cost of steam c) Availability and cost of gas.d) Availability and cost of electrical power.